Conducting Polymers: Evolution and Applications. In 1977, the field of conducting polymeric materials, also known as synthetic metals, began with the discovery that polyacetylene conducts electricity. [H. Shirakawa, E. J. Louis, A. G. Macdiarmid, C. K. Chiang, and A. J. Heeger, “Synthesis of Electrically Conducting Organic Polymers—Halogen Derivatives of Polyacetylene, (Ch)X,” Journal of the Chemical Society-Chemical Communications 16, 578-580 (1977).] Recent reviews examine efforts to incorporate conducting polymers into electronic devices, including light-emitting diodes (LEDs), electrochromic materials and structures, microelectronics, portable and large-area displays, and photovoltaics. [C. T. Chen, “Evolution of red organic light-emitting diodes: Materials and devices,” Chemistry of Materials 16(23), 4389-4400 (2004); A. P. Kulkarni, C. J. Tonzola, A. Babel, and S. A. Jenekhe, “Electron transport materials for organic light-emitting diodes,” Chemistry of Materials 16(23), 4556-4573 (2004); A. A. Argun, P. H. Aubert, B. C. Thompson, I. Schwendeman, C. L. Gaupp, J. Hwang, N. J. Pinto, D. B. Tanner, A. G. MacDiarmid, and J. R. Reynolds, “Multicolored electrochromism polymers: Structures and devices,” Chemistry of Materials 16(23), 4401-4412 (2004); D. K. James and J. M. Tour, “Electrical measurements in molecular electronics,” Chemistry of Materials 16(23), 4423-4435 (2004); C. R. Newman, C. D. Frisbie, D. A. da Silva, J. L. Bredas, P. C. Ewbank, and K. R. Mann, “Introduction to organic thin film transistors and design of n-channel organic semiconductors,” Chemistry of Materials 16(23), 4436-4451 (2004); M. L. Chabinyc and A. Salleo, “Materials requirements and fabrication of active matrix arrays of organic thin-film transistors for displays,” Chemistry of Materials 16(23), 4509-4521 (2004); and K. M. Coakley and M. D. McGehee, “Conjugated polymer photovoltaic cells,” Chemistry of Materials 16(23), 4533-4542 (2004).]
A number of polymers, such as polyphenylene, polyaniline, polythiophene, polypyrrole, polycarbazole, and polysilane, have delocalized electrons along their backbones enabling charge conduction. The conductivity rises dramatically when anions present as dopants in the polymer matrix stabilize positive charges along the chain. Each of the conducting polymers can be substituted with a variety of functional groups to achieve different properties, so new derivatives continue to be synthesized. [J. L. Brédas and R. J. Silbey, Conjugated polymers: the novel science and technology of highly conducting and nonlinear optically active materials (Kluwer Academic Publishers, Dordrecht; Boston, 1991), pp. xviii, 624 p.; and T. A. Skotheim, R. L. Elsenbaumer, and J. R. Reynolds, Handbook of conducting polymers, 2nd ed. (M. Dekker, New York, 1998), pp. xiii, 1097 p.]
A promising conducting polymer is poly-3,4-ethylenedioxy-thiophene (PEDOT) developed by scientists at Bayer AG in Germany. [Bayer, Eur. Patent 339340 (1988); B. L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, and J. R. Reynolds, “Poly(3,4-ethylenedioxythiophene) and its derivatives: Past, present, and future,” Advanced Materials 12(7), 481-494 (2000); and F. Jonas and L. Schrader, “Conductive Modifications of Polymers with Polypyrroles and Polythiophenes,” Synthetic Metals 41(3), 831-836 (1991).] It was initially designed to block the β-positions on the thiophene ring to prevent undesirable side reactions. The strategy worked and the ethylene bridge on the molecule also proved to be a good charge donor to the π-conjugated backbone, giving rise to an unusually high conductivity of 300 S/cm. [G. Heywang and F. Jonas, “Poly(Alkylenedioxythiophene)S—New, Very Stable Conducting Polymers,” Advanced Materials 4(2), 116-118 (1992).] In addition, PEDOT films in their oxidized state were observed to be extremely stable for conducting polymers and nearly transparent. [M. Dietrich, J. Heinze, G. Heywang, and F. Jonas, “Electrochemical and Spectroscopic Characterization of Polyalkylenedioxythiophenes,” Journal of Electroanalytical Chemistry 369(1-2), 87-92 (1994).] However, like other conjugated polymers that have a very rigid conformation in order to maintain electron orbital overlap along the backbone, PEDOT was found to be insoluble. Bayer addressed this problem by using a water soluble polyanion, polystyrene sulfonic acid (PSS), during polymerization as the charge-balancing dopant. The PEDOT:PSS system is marketed as BAYTRON P™ and it has good film forming capabilities, a conductivity of 10 S/cm, good transparency, and extremely good stability. In fact, the films can be heated in air over 100° C. for over 1000 hours with no major decline in conductivity. Studies demonstrate that Bayer's BAYTRON P materials (PEDOT stabilized with polystyrene sulfonate) can have increased conductivity up to 600 S/cm by adding N-methyl-pyrolidone (NMP) or other solvents that reduce screening effects of the polar solvent between the dopant and the polymer main chain. [F. Louwet, L. Groenendaal, J. Dhaen, J. Manca, J. Van Luppen, E. Verdonck, and L. Leenders, “PEDOT/PSS: synthesis, characterization, properties and applications,” Synthetic Metals 135(1-3), 115-117 (2003); and J. Y. Kim, J. H. Jung, D. E. Lee, and J. Joo, “Enhancement of electrical conductivity of poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) by a change of solvents,” Synthetic Metals 126(2-3), 311-316 (2002).] Bayer's first major customer for BATRON P was Agfa who used the material as an anti-static coating on photographic film. [F. Jonas, W. Kraffi, and B. Muys, “Poly(3,4-Ethylenedioxythiophene)—Conductive Coatings, Technical Applications and Properties,” Macromolecular Symposia 100, 169-173 (1995); Bayer, European Pat 440957 (1991); and Agfa, European Patent 564911 (1993).] Any spark generated by static electricity can expose film showing up as a bright spot on developed photos. Bayer has since enjoyed wide utilization of BAYTRON P as an electrode material in capacitors and a material for through-hole plating of printed circuit boards. [F. Jonas and J. T. Morrison, “3,4-polyethylenedioxythiophene (PEDT): Conductive coatings technical applications and properties,” Synthetic Metals 85(1-3), 1397-1398 (1997); Bayer, European Patent 533671 (1993); Bayer, European Patent 686662 (1995); Bayer, U.S. Pat. No. 5,792,558 (1996); and F. Jonas and G. Heywang, “Technical Applications for Conductive Polymers,” Electrochimica Acta 39(8-9), 1345-1347 (1994).] BAYTRON P has also been found to be suitable as a hole-injecting layer in LEDs and photovoltaics, increasing device efficiency by up to 50%. [T. M. Brown, J. S. Kim, R. H. Friend, F. Cacialli, R. Daik, and W. J. Feast, “Built-in field electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene dioxythiophene) hole injection layer,” Applied Physics Letters 75(12), 1679-1681 (1999); and G. Greczynski, T. Kugler, M. Keil, W. Osikowicz, M. Fahlman, and W. R. Salaneck, “Photoelectron spectroscopy of thin films of PEDOT-PSS conjugated polymer blend: a mini-review and some new results,” Journal of Electron Spectroscopy and Related Phenomena 121(1-3), 1-17 (2001).]
Most conducting polymer materials are formed via oxidative polymerization of aniline, pyrrole, thiophene, and their derivatives. [A. Malinauskas, “Chemical deposition of conducting polymers,” Polymer 42(9), 3957-3972 (2001).] It has not been feasible to process bulk material of these polymers into thin films since they are insoluble and non-melting, but coating techniques have been developed on substrates including plastic, glass, metal, fabric and micro- and nano-particles. There are four main approaches to form coatings of anilines, pyrroles, and thiophenes via oxidative polymerization on various materials: electropolymerization of monomers at electrodes, casting a solution of monomer and oxidant on a surface and allowing the solvent to evaporate, immersing a substrate in a solution of monomer and oxidant during polymerization, and chemical oxidation of a monomer directly on a substrate surface that has been enriched with an oxidant.
Electrochemical Oxidative Polymerization. Historically, electrochemical oxidation on different electrode materials has been the primary route to conducting polyaniline, polypyrrole films, polythiophene, and their derivatives. [E. M. Genies, A. Boyle, M. Lapkowski, and C. Tsintavis, “Polyaniline—a Historical Survey,” Synthetic Metals 36(2), 139-182 (1990); L. X. Wang, X. G. Li, and Y. L. Yang, “Preparation, properties and applications of polypyrroles,” Reactive & Functional Polymers 47(2), 125-139 (2001); E. Smela, “Microfabrication of PPy microactuators and other conjugated polymer devices,” Journal of Micromechanics and Microengineering 9(1), 1-18 (1999); Q. B. Pei, G. Zuccarello, M. Ahlskog, and O. Inganas, “Electrochromic and Highly Stable Poly(3,4-Ethylenedioxythiophene) Switches between Opaque Blue-Black and Transparent Sky Blue,” Polymer 35(7), 1347-1351 (1994); R. Kiebooms, A. Aleshin, K. Hutchison, and F. Wudl, “Thermal and electromagnetic behavior of doped poly(3,4-ethylenedioxythiophene) films,” Journal of Physical Chemistry B 101(51), 11037-11039 (1997); A. M. White and R. C. T. Slade, “Electrochemically and vapour grown electrode coatings of poly(3,4-ethylenedioxythiophene) doped with heteropolyacids,” Electrochimica Acta 49(6), 861-865 (2004); and A. Aleshin, R. Kiebooms, R. Menon, F. Wudl, and A. J. Heeger, “Metallic conductivity at low temperatures in poly(3,4-ethylenedioxythiophene) doped with PF6,” Physical Review B 56(7), 3659-3663 (1997).] Deposition takes place on an inert electrode material, which is usually platinum, but can also be iron, copper, zinc, chrome-gold, lead, palladium, different types of carbon, semiconductors, or on transparent electrodes like indium tin oxide. [D. W. Deberry, “Modification of the Electrochemical and Corrosion Behavior of Stainless-Steels with an Electroactive Coating,” Journal of the Electrochemical Society 132(5), 1022-1026 (1985); G. Mengoli, M. M. Musiani, B. Pelli, and E. Vecchi, “Anodic Synthesis of Sulfur-Bridged Polyaniline Coatings onto Fe Sheets,” Journal of Applied Polymer Science 28(3), 1125-1136 (1983); and G. Mengoli, M. T. Munari, and C. Folonari, “Anodic Formation of Polynitroanilide Films onto Copper,” Journal of Electroanalytical Chemistry 124(1-2), 237-246 (1981).] Electrochemical oxidation takes place in an acidic electrolytic solution having anions like chloride, sulfate, fluorosulfonates, and hexafluorophosphate. Deposition occurs as the potential of the electrode is cycled in a range of around −0.6 V to 1 V. Aniline and pyrrole polymerized electrochemically have conductivities ranging as high as 10 S/cm. Electrochemically deposited polythiophenes, especially the poly-3,4-ethylenedioxythiophene (PEDOT) derivative, have reached conductivities that are an order of magnitude higher, around 200 to 300 S/cm. [H. Yamato, K. Kai, M. Ohwa, T. Asakura, T. Koshiba, and W. Wernet, “Synthesis of free-standing poly(3,4-ethylenedioxythiophene) conducting polymer films on a pilot scale,” Synthetic Metals 83(2), 125-130 (1996).] Using a variety of heteropolyacids as dopants in the electrolyte solution has enabled the formation of free-standing films as opposed to coatings on electrodes. Although electrochemical polymerization produces films that are not processible, it has the advantage of making a clean product that does not need to be extracted from a solution. The experimental setup can also be coupled to physical spectroscopic techniques like visible, IR, Raman, and ellipsometry for in-situ characterization.
Solution-Based Oxidative Polymerization. Chemical oxidation of pure and substituted aniline, pyrrole, and thiophene can also take place in solution in the presence of an oxidant, for example iron(III) chloride, iron(III) toslyate, hydrogen peroxide, potassium iodate, potassium chromate, ammonium sulfate, and tetrabutylammonium persulfate (TBAP). [A. Yasuda and T. Shimidzu, “Chemical Oxidative Polymerization of Aniline with Ferric-Chloride,” Polymer Journal 25(4), 329-338 (1993); R. Corradi and S. P. Armes, “Chemical synthesis of poly(3,4-ethylenedioxythiophene),” Synthetic Metals 84(1-3), 453-454 (1997); T. Yamamoto and M. Abla, “Synthesis of non-doped poly(3,4-ethylenedioxythiophene) and its spectroscopic data,” Synthetic Metals 100(2), 237-239 (1999); F. Tran-Van, S. Garreau, G. Louarn, G. Froyer, and C. Chevrot, “Fully undoped and soluble oligo(3,4-ethylenedioxythiophene)s: spectroscopic study and electrochemical characterization,” Journal of Materials Chemistry 11(5), 1378-1382 (2001); D. Hohnholz, A. G. MacDiarmid, D. M. Sarno, and W. E. Jones, “Uniform thin films of poly-3,4-ethylenedioxythiophene (PEDOT) prepared by in-situ deposition,” Chemical Communications (23), 2444-2445 (2001); A. Pron, F. Genoud, C. Menardo, and M. Nechtschein, “The Effect of the Oxidation Conditions on the Chemical Polymerization of Polyaniline,” Synthetic Metals 24(3), 193-201 (1988); M. Angelopoulos, G. E. Asturias, S. P. Ermer, A. Ray, E. M. Scherr, A. G. Macdiarmid, M. Akhtar, Z. Kiss, and A. J. Epstein, “Polyaniline—Solutions, Films and Oxidation-State,” Molecular Crystals and Liquid Crystals 160, 151-163 (1988); Y. Cao, A. Andreatta, A. J. Heeger, and P. Smith, “Influence of Chemical Polymerization Conditions on the Properties of Polyaniline,” Polymer 30(12), 2305-2311 (1989); J. C. Chiang and A. G. Macdiarmid, “Polyaniline—Protonic Acid Doping of the Emeraldine Form to the Metallic Regime,” Synthetic Metals 13(1-3), 193-205 (1986); and I. Kogan, L. Fokeeva, I. Shunina, Y. Estrin, L. Kasumova, M. Kaplunov, G. Davidova, and E. Knerelman, “An oxidizing agent for aniline polymerization,” Synthetic Metals 100(3), 303-303 (1999).]
The strongest of the acid anions (e.g., chloride) often result in the highest conductivities. Chemical oxidation has been shown to produce polymers with comparable conductivities to electrochemically deposited films using the same oxidant. [J. A. Walker, L. F. Warren, and E. F. Witucki, “New Chemically Prepared Conducting Pyrrole Blacks,” Journal of Polymer Science Part a-Polymer Chemistry 26(5), 1285-1294 (1988).] Generally, polar solvents like alcohols and acetonitrile are used and reactions take place over a range of oxidant concentrations and temperatures including 0 to 80° C., under acidic or basic conditions. [L. Yu, M. Borredon, M. Jozefowicz, G. Belorgey, and R. Buvet, Journal of Polymer Science Part A-Polymer Chemistry 10, 2931 (1987).] However, these parameters can influence the yield of polymerization and the conductivity of the final product. Solution-based chemical oxidation produces powder precipitates that are packed into the form of a tablet and then characterized since they are generally insoluble and do not melt. The polymerized material is usually rinsed in water, methanol, or acetonitrile to remove unreacted oxidant. Improved conductivities can be obtained after rinsing the material in a solution containing anionic dopants like hydrochloric acid (HCl) or dissolved nitrosonium hexafluorophosphate (NOPF6). Chemical polymerization of aniline and pyrrole results in conductivities as high as 10 or 50 S/cm, although using TBAP was reported to give polyaniline with a conductivity of 400 S/cm. PEDOT can often have a conductivity of up to 200 or 300 S/cm. Forming a film with these materials has been demonstrated by immersing a substrate in the reaction solution during polymerization.
Polyaniline was also synthesized via the Ullmann reaction using p-bromoaniline as a precursor, but a low conductivity of only 10−6 S/cm was measured. [F. Ullmann, Ber Dtsch Chem Ges 36, 2382-2384 (1903).] Interfacial polymerization of pyrrole using an aqueous oxidant solution and an organic phase with dissolved monomer achieved free-standing thin films with conductivities of as high as 50 S/cm. [M. Nakata, M. Taga, and H. Kise, “Synthesis of Electrical Conductive Polypyrrole Films by Interphase Oxidative Polymerization—Effects of Polymerization Temperature and Oxidizing-Agents,” Polymer Journal 24(5), 437-441 (1992).]
Oxidative Polymerization of Vapor-Phase Monomers. Dipping a substrate material into a solution containing oxidants like FeCl3 and Fe(OTs)3 and allowing it to dry yields an oxidant-enriched substrate that can provide a reactive surface that polymerizes volatile monomers including aniline, pyrrole, thiophene, and their derivatives. [B. Winther-Jensen, J. Chen, K. West, and G. Wallace, “Vapor phase polymerization of pyrrole and thiophene using iron(III) sulfonates as oxidizing agents,” Macromolecules 37(16), 5930-5935 (2004); J. Kim, E. Kim, Y. Won, H. Lee, and K. Suh, “The preparation and characteristics of conductive poly(3,4-ethylenedioxythiophene) thin film by vapor-phase polymerization,” Synthetic Metals 139(2), 485-489 (2003); and B. Winther-Jensen and K. West, “Vapor-phase polymerization of 3,4-ethylenedioxythiophene: A route to highly conducting polymer surface layers,” Macromolecules 37(12), 4538-4543 (2004).] A conductivity of 50 S/cm has been achieved using this method for polypyrrole and 500 to 1000 S/cm has been reported for PEDOT. Oxidant pretreatment has been extended to glass substrates, polymers, fabrics, and-even individual fibers. [S. N. Tan and H. L. Ge, “Investigation into vapour-phase formation of polypyrrole,” Polymer 37(6), 965-968 (1996); A. Mohammadi, I. Lundstrom, and O. Inganas, “Synthesis of Conducting Polypyrrole on a Polymeric Template,” Synthetic Metals 41(1-2), 381-384 (1991); and C. C. Xu, P. Wang, and X. T. Bi, “Continuous Vapor-Phase Polymerization of Pyrrole .1. Electrically Conductive Composite Fiber of Polypyrrole with Poly(P-Phenylene Terephthalamide),” Journal of Applied Polymer Science 58(12), 2155-2159 (1995).] Pretreating a surface with iodine vapors present a solvent-less polymerization process for polypyrrole, but only low conductivities ranging from 10−7 to 10−1 S/cm result. [S. L. Shenoy, D. Cohen, C. Erkey, and R. A. Weiss, “A solvent-free process for preparing conductive elastomers by an in situ polymerization of pyrrole,” Industrial & Engineering Chemistry Research 41(6), 1484-1488 (2002).]
Another solvent-less deposition process has been demonstrated for polypyrrole that eliminates the step of dip-coating a substrate in an oxidant solution, although it is restricted to metallic substrates. The surface of metals including copper, iron, gold, and palladium can be converted into salts by exposing them to vapors of chlorine, iodine, bromine and acids with F−, NO3−, SO42−, and ClO4− anions. Subsequent introduction of monomer vapor results in polymer films with conductivities of around 1 to 10 S/cm. [G. Serra, R. Stella, and D. De Rossi, “Study of the influence of oxidising salt on conducting polymer sensor properties,” Materials Science & Engineering C-Biomimetic Materials Sensors and Systems 5(3-4), 259-263 (1998); A. Nannini and G. Serra, “Growth of Polypyrrole in a Pattern—a Technological Approach to Conducting Polymers,” Journal of Molecular Electronics 6(2), 81-88 (1990); F. Cacialli and P. Bruschi, “Site-selective chemical-vapor-deposition of submicron-wide conducting polypyrrole films: Morphological investigations with the scanning electron and the atomic force microscope,” Journal of Applied Physics 80(1), 70-75 (1996); and P. Bruschi, F. Cacialli, A. Nannini, and B. Neri, “Low-Frequency Resistance Fluctuation Measurements on Conducting Polymer Thin-Film Resistors,” Journal of Applied Physics 76(6), 3640-3644 (1994).]
Non-Oxidative Polymerization of Vapor-Phase Monomers. Vacuum deposition of polyaniline films using evaporation has been demonstrated yielding conductivities of about 10−4 S/cm. [T. L. Porter, K. Caple, and G. Caple, “Structure of Chemically Prepared Freestanding and Vacuum-Evaporated Polyaniline Thin-Films,” Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 12(4), 2441-2445 (1994); and T. R. Dillingham, D. M. Cornelison, and E. Bullock, “Investigation of Vapor-Deposited Polyaniline Thin-Films,” Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 12(4), 2436-2440 (1994).] Solid polyaniline can be sublimed at temperatures of about 400° C. and pressures of 10−5 or 10−6 Torr. Compositional analysis using XPS showed a carbon-to-nitrogen ratio of about 6:1 for the source material. A slightly higher C/N ratio in the deposited film indicates that short oligomers preferentially sublime.
Plasma-enhanced chemical vapor deposition (PECVD) has been investigated as a method of depositing aniline, thiophene and parylene-substituted precursors yielding low conductivities of only 10−4 S/cm due to ring breakage and other imperfections caused by the high energies inherent with plasma. [C. J. Mathai, S. Saravanan, M. R. Anantharaman, S. Venkitachalam, and S. Jayalekshmi, “Characterization of low dielectric constant polyaniline thin film synthesized by ac plasma polymerization technique,” Journal of Physics D-Applied Physics 35(3), 240-245 (2002); K. Tanaka, K. Yoshizawa, T. Takeuchi, T. Yamabe, and J. Yamauchi, “Plasma Polymerization of Thiophene and 3-Methylthiophene,” Synthetic Metals 38(1), 107-116 (1990).; L. M. H. Groenewoud, G. H. M. Engbers, R. White, and J. Feijen, “On the iodine doping process of plasma polymerised thiophene layers,” Synthetic Metals 125(3), 429-440 (2001); and L. M. H. Groenewoud, A. E. Weinbeck, G. H. M. Engbers, and J. Feijen, “Effect of dopants on the transparency and stability of the conductivity of plasma polymerised thiophene layers,” Synthetic Metals 126(2-3), 143-149 (2002).] Another conjugated monomer, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), fared much better using PECVD with reported conductivities as high as 50 S/cm. [M. Murashima, K. Tanaka, and T. Yamabe, “Electrical-Conductivity of Plasma-Polymerized Organic Thin-Films—Influence of Plasma Polymerization Conditions and Surface-Composition,” Synthetic Metals 33(3), 373-380 (1989).]
Thermally activated hot-wire CVD has been used to polymerize aniline and vinyl-containing monomers including phenylenevinylene and vinylcarbazole, but no conductivities of these materials have been reported. [G. A. Zaharias, H. H. Shi, and S. F. Bent, “Hot Wire Chemical Vapor Deposition as a Novel Synthetic Method for Electroactive Organic Thin Films,” Mat. Res. Soc. Symp. Proc. 814, I12.19.11-I12.19.16 (2004); K. M. Vaeth and K. F. Jensen, “Chemical vapor deposition of thin polymer films used is polymer-based light emitting diodes,” Advanced Materials 9(6), 490-& (1997); K. M. Vaeth and K. F. Jensen, “Transition metals for selective chemical vapor deposition of parylene-based polymers,” Chemistry of Materials 12(5), 1305-1313 (2000); and M. Tamada, H. Omichi, and N. Okui, “Preparation of polyvinylcarbazole thin film with vapor deposition polymerization,” Thin Solid Films 268(1-2), 18-21 (1995).] Photo-induced CVD was used to couple thiophene monomers that were halogenated with bromine and chlorine at the 2- and 5-positions. [T. Sorita, H. Fujioka, M. Inoue, and H. Nakajima, “Formation of Polymerized Thiophene Films by Photochemical Vapor-Deposition,” Thin Solid Films 177, 295-303 (1989).] Although the monomers only coupled at the halogenated positions, only relatively low conductivities of 10−13 S/cm were achieved. However, dibrominated PEDOT polymerizes upon heating and makes films with a conductivity of 20 S/cm. [H. Meng, D. F. Perepichka, M. Bendikov, F. Wudl, G. Z. Pan, W. J. Yu, W. J. Dong, and S. Brown, “Solid-state synthesis of a conducting polythiophene via an unprecedented heterocyclic coupling reaction,” Journal of the American Chemical Society 125(49), 15151-15162 (2003).]
Norborene and a ruthenium(IV)-based catalyst volatilized in a chamber under low pressure resulted in a film of polynorborene on a silicon substrate. [H. W. Gu, D. Fu, L. T. Weng, J. Xie, and B. Xu, “Solvent-less polymerization to grow thin films on solid substrates,” Advanced Functional Materials 14(5), 492-500 (2004).] The monomer undergoes a ring-opening metathesis polymerization mechanism when contacted by the catalyst. [T. M. Trnka and R. H. Grubbs, “The development of L2X2Ru=CHR olefin metathesis catalysts: An organometallic success story,” Accounts of Chemical Research 34(1), 18-29 (2001).] The technique was repeated based on the same polymerization mechanism using 1,3,5,7-cyclooctatetetraene to form polyacetylene. [H. W. Gu, R. K. Zheng, X. X. Zhang, and B. Xu, “Using soft lithography to pattern highly oriented polyacetylene (HOPA) films via solvent-less polymerization,” Advanced Materials 16(15), 1356 (2004).] The deposition is slow and an electrical conductivity was not reported.
As described above, existing synthesis techniques for conducting polymers preclude their deposition on some substrates like paper or on top of other materials that are incompatible with solutions-based processing. This fact generally limits their application in electronic devices to use as a bottom-electrode layer or as a coating on a bottom electrode to facilitate better hole injection. The development of a robust vapor-deposition technique for conducting polymers that preserves their high conductivity and is compatible with moisture-sensitive, temperature-sensitive, and mechanically fragile surfaces is needed to broaden their utilization, enabling both improved efficiencies in existing devices and development of new devices on unconventional substrates. Such a robust vapor-deposition technique is described herein.